The present invention is directed to a chuck adapted to test electrical and/or optical components on a device-under-test (DUT).
Guarding systems suitable to reduce leakage currents during low current measurements are well known and discussed extensively in the technical literature. See, for example, an article by William Knauer entitled “Fixturing for Low Current/Low Voltage Parametric Testing” appearing in Evaluation Engineering, November, 1990, pages 150-153. Probe stations employing such a guarding system typically route a test signal to selected contact pads on the device-under-test (DUT) and route a guard signal to electrically conductive material surrounding the DUT on several sides, separated from the device-under-test by dielectric material (e.g., air). The guard signal preferably closely approximates the test signal or otherwise follows the test signal, thus reducing electromagnetic leakage currents that might otherwise occur.
Frequently, such probe stations also provide an electrically conductive enclosure around the perimeter of the probe station connected to a shield potential. The shield potential is typically connected to earth ground, instrumentation ground, or some other suitable potential. See, for example, Peters et al., U.S. Pat. No. 6,002,263.
To provide effective guarding and shielding for probe stations, a multi-stage chuck upon which the device-under-test rests during testing may likewise be used. The upper stage of the chuck, which supports the device-under-test, typically includes an electrically conductive metal layer through which the test signal may be routed. A middle stage and a lower stage of the chuck similarly include an electrically conductive metal layer to which a guard signal and a shield signal may be imposed, respectively. In this fashion, a device-under-test resting on such a multistage chuck may be both guarded and shielded from below. Some probe stations also provide for guarding from the sides and from above.
Many electrical devices, in particular semiconductor based devices, include both electrical components and optical components. Some optical components receive an optical signal from an optical source and convert the received optical signal into an electrical signal, e.g., a photo-detector. Other optical components convert an electrical signal into an optical signal, e.g., a light-emitting-diode. Yet other optical components may include multiple optical and/or electrical components. Frequently, a probe station may be used to test the electrical components.
Unfortunately, the aforementioned probe stations are not suitable for testing optical components because there is no optical path through the chuck itself. Accordingly, a different type of chuck, namely an optical chuck, is used for testing devices that include optical components. An optical chuck typically includes an central optically transparent medium over which the device-under-test is supported. For example, an optical signal from a light source may be directed toward the device-under-test from below, above, or to the side of the optical chuck, and a probe or connector used to sense the resulting electrical output from the device-under-test. Similarly for example, a probe or connector may be used to provide an electrical source to the device-under-test, and an optical sensing device located below, above, or to the side of the optical chuck to sense the resulting optical output from the device-under-test. Accordingly, the probe station is used to provide a shielded environment from exterior electromagnetic noise so that the input-output characteristics of an optical device-under-test may be performed. It may be observed that the testing of the optical components on the device-under-test is performed by observing the input and output characteristics of the device which normally have significant voltage and/or current levels (or optical power) making measurements easily performed.